The aim of this investigation is to provide a comparative biochemical study, at the molecular level, of the oxygen-carrying pigments. Enormous efforts have been applied to hemoglobin in many laboratories. We are attempting to obtain complementary information for the non-heme oxygen carriers, hemerythrin and hemocyanin, in the hopes of elucidating relationships between molecular structure and biological function, in this case, oxygen binding. Our investigations will continue in the following directions: (1) further studies of primary structure, particularly to identify mutant substitutions in sequence and to make phylogenetic comparisons; (2) identification of the residues in the protein that provide ligands to the two irons at each active site; (3) more detailed elucidation of the electronic state of the irons at the active sites; (4) preparation of crystals for X-ray diffraction studies; (5) molecular delineation of the interface areas involved in subunit interactions.